1. Field of the Invention
The present invention relates generally to scroll compressors and, more particulary, to a method of precisely working at least one of stationary and orbiting scroll members to be assembled to form a pump section of a scroll compressor.
2. Description of the Prior Art
Japanese Unexamined Patent Publication No. 1-187,388 shows the structure of a pump section of a scroll compressor. Such pump section of a scroll compressor will be described hereinunder with reference to FIG. 9.
A stationary scroll member 21 has a spiral wrap 21a upstanding from one surface of a substantially circular base or end plate 21b such that the inner and outer peripheral surfaces of the wrap 21a are perpendicular to the end plate 21b. An orbiting scroll member 22 has a spiral wrap 22a upstanding from one surface of a substantially circular base or end plate 22b such that the inner and outer peripheral surfaces of the wrap 22a are perpendicular to the end plate 22b. The two scroll members 21 and 22 are assembled such that the spiral wraps 21a and 22a are meshing engagement with each other and cooperate to define a compression chamber 30.
The orbiting scroll member 22 is provided with a cylindrical bearing portion 22c extending from the other surface of the end plate 22b. Radial key grooves are formed in the other surface of the end plate 22b radially outwardly of the bearing portion 22c and engaged by axial projections 24a of an oldhams ring 24. The outer periphery of the stationary scroll member 21 is secured by bolts 26 to an outer periphery of a frame 23 having an end which is recessed to receive therein the orbiting scroll member 22 and the oldhams ring 24. The other end of the frame 23 is shaped to provide a tubular bearing 23a through which a crank shaft 25 extends from an electric motor (not shown) and has an eccentric end portion 25a received in the bearing portion 22c. The frame 23 has formed therein radial key grooves 23b which are engaged with further projections 24b extending from the oldhams ring 24 in its axial direction opposite to the axial direction in which the projections 24a extend from the oldhams ring 24. The eccentric portion 25a of the crank shaft 25 and the projections 24a and 24b of the oldhams ring 24 and the associated key grooves 22d and 23b cooperate to convert the rotation of the crank shaft 25 into an orbiting motion of the orbiting scroll member 22, so that the compression chamber 30 is moved in the pump section radially inwardly to suck gas (such as refrigerant gas) through a suction port 21-2 while the volume of the compression chamber 30 is gradually reduced to compress the thus sucked gas. When the compression chamber 30 is moved to the radially innermost position, the chamber 30 is communicated with a discharge port 21-1 so that the compressed gas is discharged out of the pump section through the discharge port 21-1.
In a hermetic scroll compressor, the aforedescribed pump section and the electric motor (not shown) are housed in a hermetic container which has a gas inlet pneumatically connected to the suction port so that refrigerant gas is sucked from a refrigeration cycle (not shown) through the gas inlet in the hermetic container and through the suction port 21-2 into the compression chamber 30. The hermetic container has formed therein a gas outlet communicated with the discharge port 21-1 of the pump section through the space defined in the container. Accordingly, compressed refrigerant gas is discharged from the pump section through the discharge port 21-1 and flows through the interior of the hermetic container to the gas outlet therein. Accordingly, the end plate 21b of the stationary scroll member 21 is designed to be of a large thickness to provide a sufficient rigidity to prevent the stationary scroll end plate 21b from being deformed by pressure exerted axially thereto by the compressed refrigerant gas discharged from the pump section through the discharge port 21-1 into the interior of the hermetic container.
On the other hand, the end plate 22b of the orbiting scroll member 22 is designed to have as small thickness and weight as possible so as to minimize the inertia of the orbiting scroll member which would cause vibration of the compressor when the orbiting scroll member is driven to make an orbiting motion by the eccentric portion 25a of the crank shaft 25. For this reason, the structure of the orbiting scroll end plate 22b is of a low rigidity.
An example of the methods of working (machining) the orbiting scroll member 22 is schematically shown in FIG. 10. A working machine or machine tool is shown to have a jaw chuck comprising gripping fingers 27 and a back-up plate 28. The chuck is designed such that the gripping fingers 27 are forcibly moved radially inwardly into gripping engagement with the outer peripheral surface of the circular end plate 22b of a semifinished orbiting scroll member 22 which has been prepared, for example, by casting a metal. While the scroll member 22 is so held by the gripping fingers 27 and the back-up plate 28, a cutting or grinding tool is brought into working engagement with the side of the end plate 22b remote from the back-up plate 28 to effect machining on that side of the end plate.
In the working method shown in FIG. 10, however, the gripping force radially inwardly exerted to the end plate 22b axially deforms the end plate away from the back-up plate 28 in a manner shown by broken lines in FIG. 10. It will therefore be appreciated that the machining of the orbiting scroll member 22 is conducted with the end plate 22b deformed in one axial direction thereof. Accordingly, when the radially inward gripping force is removed from the outer peripheral surface of the end plate 22b of the orbiting scroll member 22 when the machining is finished, the end plate 22b tends to recover to its initial shape. This working or machining method, therefore, fails to provide a precise flatness of the machined side of the orbiting scroll end plate 22b. In addition, the working method shown in FIG. 10 has a further disadvantage that the end plate 22b of the orbiting scroll member 22 cannot be kept in close contacting relationship with the back-up plate 28 because the end plate 22b is deformed axially away from the back-up plate 28, with a result that a variation is liable to occur in the precise thickness of the machined end plate 22b of the orbiting scroll member 22.
The performance of a scroll compressor is greatly influenced by the rate of gas leakage through gaps between side faces of meshed spiral wraps, between the end face of each wrap and an associated bottom face of an associated grooved bottom and between the end plates of stationary and orbiting scroll members. The scroll members can be machined with a substantial precision to obtain desired shapes of spiral wraps, desired heights thereof and desired depths of grooved bottoms. However, the flatness of each of the stationary and orbiting scroll members depends upon how the scroll member is gripped by a jaw chuck. The degree of the flatness of the end plate of each of stationary and orbiting scroll members directly influences the gap between the two end plates. With the structure of the conventional orbiting scroll member and with the conventional method of working orbiting scroll member, it has been quite difficult to obtain a desired precision of the flatness of the end plate of the orbiting scroll member. For this reason, a relatively thicker scroll end plate had to be employed to assure a desired rigidity of the end plate notwithstanding the fact the thicker end plate disadvantageously increases the weight thereof with a resultant increase in the inertia thereof and thus in an increase in the vibration of the compressor.
The above discussion is also applicable to the case in which the pump section is not accommodated in a hermetic container. In such a case, the discharge port 21-1 formed in the end plate 21b of the stationary scroll member 21 is connected directly to a refrigeration cycle, so that the pressure of the compressed gas discharged from the pump section does not act on the end plate 21b of the stationary scroll member 21. Accordingly, the stationary scroll end plate 21b should preferably be designed to be of as smaller thickness as possible to reduce its weight and inertia and thus to decrease the vibration of the compressor.